The present invention relates generally to waveform display systems and, in particular, to automotive engine analyzers including such display systems.
Digital engine analyzers having digital oscilloscope display screens are well known, one such analyzer and the waveform display system therefor being disclosed in copending U.S. application Ser. No. 08/629,484, filed Apr. 10, 1996, and entitled "System for Reconfiguring Oscilloscope Screen in Freeze Mode", the disclosure of which is incorporated herein by reference. Under some circumstances, a signal displayed on the oscilloscope screen of such an analyzer can be clipped or otherwise altered due to rails in the oscilloscope circuitry, resulting in a misleading waveform display. For example, the engine analyzer disclosed in the aforementioned application includes an analog-to-digital converter ("ADC") which has rails, i.e., the voltages at which the ADC reaches its full negative or positive count. Any acquired waveform voltage exceeding the rail voltage level will cause no further change in the output of the ADC. When this occurs, it is said that the ADC is "railed" and that the acquired waveform is clipped at the rail level. It is sometimes possible for the operator to view a waveform display which is clipped in this manner, without being aware that the waveform is clipped. This may be particularly true in the case of some waveforms, such as square waves, wherein it is difficult to determine whether or not the waveform is clipped.
This can be illustrated in FIGS. 2-5. FIG. 2 shows a zero to 3.5-volt sine wave acquired and plotted on a 5-volt scale. In the engine analyzer of the aforementioned application, the 5-volt scale uses a hardware channel which has a range of almost 32 volts, i.e., the ADC reaches its maximum count when the acquired incoming signal is close to 32 volts. FIG. 2 shows the same waveform acquired and plotted on a 2-volt scale. This scale uses a hardware channel which has a much smaller range of about 3.2 volts. Thus, as can be seen in FIG. 3, the 3.5-volt sine wave extends beyond the plot area on a 2-volt scale. FIG. 4 shows the same signal on a 2-volt scale with a 1.2-volt offset (i.e., the scale extends from 1.2 volts to 3.2 volts). As can be seen, the top of the waveform is clipped as it bumps up against the 3.2-volt rail level of the ADC. Since this rail level is at the top of the scale, it would be very difficult for an operator to determine whether the top of the waveform is removed simply because it is extending beyond the scale (as in FIG. 3) or because it is being clipped by encountering a rail level in the oscilloscope circuitry.
Also, engine analyzers of the type described in the aforementioned application have the capability of operating in a freeze mode, wherein a currently acquired and displayed waveform can be frozen on the screen. This analyzer is also capable of changing the display format in the freeze mode. FIG. 5 illustrates the waveform of FIG. 3 after the screen has been frozen and the display scale switched to the 5-volt scale. Since the waveform data was captured in the 2-volt scale (which has a 3.2-volt rail), switching to the 5-volt scale in freeze mode does not extend the range of the scale, as it would in live mode (FIG. 2). Thus, the waveform in FIG. 5 is clipped at the 3.2-volt rail level. While this is fairly apparent with the sine waveform of FIG. 5, it might be very difficult to determine with another waveform, such as a square wave.